1. Field of the Invention
This invention relates generally to microvalves. More particularly, this invention relates to piezoelectrically operated microvalves and their use in fuel cell systems.
2. Description of Related Art
Although discovered more than a century ago, fuel cells have received much recent attention due to their promise of compact, efficient, and clean power generation. A fuel cell is an electrochemical device, which directly converts chemical energy stored in a fuel (e.g. hydrogen) and an oxidizer (e.g. oxygen) directly into electrical energy. The reactant gases flow through a labyrinth of flow channels that are lined with catalyzed electrodes which are sandwiched about an electrolytic material. The fuel cell is classified in terms of the electrolyte, e.g. polymer electrolyte membrane (PEM), solid oxide (SOFC), molten carbonate, alkaline or phosphoric acid. The Fuel Cell Handbook (Hirschenhofer, et. al., 1999) published by the Department of Energy (DOE) describes the various types of fuel cells (eg. alkaline fuel cell, molten carbonate fuel cell, phosphoric acid fuel cell, polymer electrolyte membrane fuel cell (PEMFC), solid oxide fuel cell), each of which are in various stages of development. All fuel cells share some common advantages over other fossil energy power sources: 1) Potential for higher efficiency compared to conventional heat engines 2) Zero or near zero emissions of NOx and 3) Simplistic designs that are ideal for mass production. As a result, the interest in fuel cells for automotive power, stationary power, portable power, and military applications is very high. Most of the major automobile manufacturers have, or are developing, prototype vehicles powered by a PEMFC. Other development efforts are ongoing for using the PEMFC for portable, residential, and military power applications. However, some technical and cost issues need to be addressed as well as the establishment of a fuel distribution network before fuel cells will be widely accepted into the commercial market.
Independent of the particular fuel cell technology employed, poor distribution of either fuel or oxidant throughout the fuel cell can cause many problems, including poor fuel conversion efficiency, “hot spots” within the fuel cell, decreased fuel cell life, and reduced cell voltage. Similarly, poor fuel distribution between fuel cells in a fuel cell stack can cause poor fuel conversion efficiency, overall poor stack operation and even damage to the fuel cell.
In order to provide a clearer understanding of the operation of fuel cells, we will discuss here the PEM fuel cell, one of the more widely used types of fuel cells. The PEM fuel cell readily oxidizes hydrogen (which releases electrons to an external load), and reduces oxygen (which reacquires the electrons from the load). A single cell consists of an anode flow field plate, a membrane electrode assembly (MEA) and a cathode flow field plate. The MEA contains a layer of catalyst on each side of the ion exchange membrane to induce the desired electrochemical reactions. The hydrogen fuel flows through the channels in the anode flow field plates and reacts with the catalyst to form cations, which then migrate from the anode to the cathode through the ion exchange membrane. The oxygen flows through the channels in the cathode flow field plates and reacts with the catalyst layer to form anions, which combine with the H+ cations from the anode to form water:Anode half reaction: 2H2→4H++4e−  (1)Cathode half reaction: O2+4H++4e−→2H2O  (2)
Typically, two or more cells are connected in electrical series to form a fuel cell stack. The stack includes inlet manifolds for the fuel and oxidant, and outlet manifolds for removing any unused reactant gas plus the generated water. Other fuel cell technologies will have different half-cell reactions. It is the transfer of electrons from the hydrogen to the oxygen (through an external load) that produces useful electricity. The maximum voltage, E, produced by each cell can be calculated using the Nernst Equation. For the PEM fuel cell:
                    E        =                              E            o                    +                                    RT                              2                ⁢                F                                      ⁢                          ln              ⁡                              (                                                                            P                                              H                        2                                                              ⁢                                                                  P                                                  O                          2                                                                                                                                                P                                                                        H                          2                                                ⁢                        O                                                              ⁢                                                                  P                        o                                                                                            )                                                                        (        3        )            where E is the equilibrium potential, Eo is the standard potential for an H2/O2 fuel cell taken at a pressure of Po, R is the universal gas constant, Px is the partial pressure of gas “x”, and F is Faraday's constant.
This ideal equilibrium voltage in practice is reduced by other quantities called Over Potentials. The magnitude of these over potentials depends on such things as the cell's internal electrical resistance, concentration gradients near the cell's electrodes, or the activation energies needed to initiate the desired electrochemical reactions. Notice that the cell's developed potential is limited by the concentrations (or pressures) of reactants at the electrode surface. This clearly indicates that if the concentration of one or both of the reactants were to drop to a low value, the cell's voltage would also drop. Again, concentrations may drop because of the build up of reaction products (such as liquid water as in the case of PEM fuel cells), or because of non-uniform operation of the cell's membrane. Defects in the membrane can also cause low concentrations of needed reactants.
In actual operation, individual fuel cells are combined into a stack such that appreciable voltage can be obtained. Each cell is electrically wired in series and reactants and effluents flow through common manifolds. The cells are controlled as a unit by specifying the pressure and flow through these manifolds. The desired operating point for a system is determined based on competing factors such as desired life cycle, operating efficiency, and capital versus operating costs. Every cell within the stack is not identical and inherently performs slightly differently given the same input conditions since there are variances in the ohmic polarizations, catalyst loadings, and diffusion constants, for example. In addition, both the PEFC and the SOFC use a fixed flow field and inlet fuel flow manifold design. The cell geometry within a fuel cell stack provides many flow paths for reactants to flow, which inherently creates flow distribution problems that can potentially cause inefficiencies or even damage to individual cells. There is currently no mechanism to monitor or control individual fuel cells within a stack system.
In particular, the existing PEM fuel cell stack systems experience problems associated with poor distribution of reactant fuel across the membrane surface, which inherently causes a decrease in the membrane life cycle. In addition, excess reactant flow is commonly used to correct for insufficient flow distribution across the membrane surface, and this causes a decrease in fuel utilization efficiency. To further illustrate the problems that arise due to mal-distribution of reactants, consider a PEM stack employing cells with parallel flow channels. Problems arise when the generated water vapor condenses onto the channel surfaces thereby blocking or restricting the flow through the channel, and the amount of hydrogen reacting with the catalyst. When this happens, the associated cell will perform poorly (generate low voltage and current) and cause the cell to overheat locally at the misbehaved channel. Damage can result if the stack is operated for too long at these conditions. These situations can arise at different operating points, depending on the temperature, pressure, anode and cathode flow rates, and power draw from the stack. This makes the design of the channel and manifold geometry difficult, especially if the system is to be used under a wide range of operating conditions. Additionally, the aforementioned situations can go unnoticed by typical control strategies that only monitor the overall stack voltage. Simply correcting for lost voltage due to poor flow distribution by boosting the flow (and therefore voltage) across other cells may ignore the potentially catastrophic failure of the “bad spot” in the problem cell. What is needed, it is believed is an apparatus and process that permits improved flow control over the PEM fuel cell system. The inventors believe that the novel technique of integrated flow distribution and control for individual fuel cells within the stack that is proposed will answer this need.
Currently, fuel cell designers have tried to manage the concentrations of needed reactants by imposing relatively large pressure drops across the flow channels across the total cell, thereby attempting to ensure adequate flow in each individual channel within the cell.
Microelectromechanical or MEMS technology offers the promise of cheap mass-production of compact electromechanical systems for embedded sensing and control applications such as those required for reliable and efficient fuel cell system operation. It is hoped that such devices would allow localized flow controllers to be embedded directly into the flow channels of the fuel cell, at a fraction of the cost of conventional technology and with no size penalty. Therefore, NETL is investigating MEMS for controlling flow within the stack to increase performance and decrease life cycle cost of fuel cell systems. There is a need for improved flow control within the fuel cell stack.
Minimizing cost is very important, yet the lowest cost electricity from a PEMFC occurs at higher nominal cell potential where the fuel cell is more efficient, and the savings on fuel costs offset the capital cost of additional cells [6]. However, for automotive applications, for example, the size and weight of additional cells and the stack performance at higher current density conditions can have a significant impact on cost. Kazmin [7] developed a model to determine the minimal efficiency one can operate a PEMFC in order to make the fuel cell system technically and economically feasible. In Kazmin's model the minimum stack operating efficiency that provides acceptable economics is mainly affected by the fuel cost and cell efficiency. To maximize operating efficiency the parasitic power requirements must be minimized and each cell within the stack must operate as designed.
In addition, reducing the parasitic power requirements can be achieved through the use of low power control devices, and reducing the amount of peripheral hardware required to control the stack. Furthermore, maximum efficiency can only be achieved through effective thermal and water management, and proper gas flow distribution to each cell within the stack. A recent development effort for 1 kW class PEMFC power system shows how the cell-to-cell voltage distribution within a stack can be improved through effective thermal and flow management techniques. Lee noted that in a single cell, the optimum fuel flow rate is 1.3 times the required fuel stoichiometry, but in a stack the fuel flow rates must be set much higher to ensure adequate fuel flow to each cell within the stack. This is a typical approach to ensure adequate fuel flow to each cell, and the unused fuel is re-circulated back to the inlet. A variation of this technique, named the anode water removal technique, requires an increased pressure drop across the cell and increased amounts of unused fuel to re-circulate [8]. More importantly, as shown by Lee, inadequate distribution of fuel cell-to-cell can cause irreparable damage to the MEA and lead to cell failure.
An alternate approach that is under investigation at the National Energy Technology Laboratory (NETL) is to integrate small micro-valves along the fuel feed manifold of the stack to control the fuel flow to each cell within the stack. By providing cell-to-cell flow control, the additional flow required for stack operation can potentially be reduced below the stated 1.3 times the stoichiometry, which improves fuel efficiency. More importantly, the overall efficiency can be improved by optimizing the performance of each cell within the stack. For example, the data produced by Lee [3] with the improved control technique still has greater than 10% variation of cell-to-cell voltages within the stack at relatively low current density conditions. It is anticipated that local flow control can minimize this cell-to-cell voltage variation and also reduce the required peripheral hardware, thus maximizing reliability and efficiency, and reducing cost. In addition, by using the micro-systems manufacturing techniques the additional capital cost for the micro-valves could be low.